1. Field of the Invention
Embodiments of the present invention relate generally to methods and systems for forming conductive vias in a substrate, such as, for example, a semiconductor die, a full or partial semiconductor wafer, or a printed circuit board.
2. Background of Related Art
Circuit boards (often referred to as “printed circuit boards,” “wiring boards,” or “printed wiring boards”) are used in electronic devices to mechanically hold and to provide electrical communication between the electrical components thereof. Electrical components such as transistors, resistors, capacitors, and semiconductor devices (e.g., electronic signal processors and memory devices) may be soldered into a circuit board and electrically interconnected by electrically conductive signal traces formed within or on the surfaces of the circuit board.
Semiconductor dies, semiconductor die packages, and semiconductor device modules may be provided that include a number of electrically conductive elements, which may be structurally and electrically coupled to corresponding conductive elements formed on a substrate such as a printed circuit board. Such dies, packages, and modules may include, by way of example only, microprocessors, dynamic random access memory (DRAMs), static random access memory (SRAMs), non-volatile memory such as flash memory, and application specific integrated circuits (ASICs), and may each have hundreds or thousands of electrically conductive elements requiring electrical interconnection with the conductive elements of a circuit board. The electrically conductive traces in the circuit board are used to interconnect the semiconductor devices and the other electrical components.
To provide electrical communication between various devices and components in a microelectronic device or system, it may be necessary to provide electrical communication through a substrate (such as, for example, a semiconductor die, or a full or partial semiconductor wafer, a semiconductor device package, or a printed circuit board) between conductive elements that are disposed in different planes. Electrically conductive vias are used, conventionally, to establish such electrical communication. An electrically conductive via typically includes a hole at least partially penetrating the substrate that is at least partially filled or lined with conductive material.
Various methods for forming conductive vias in substrates have been proposed in the art. Generally, an aperture (commonly known as a “via hole”) is formed at least partially through the substrate, typically in a direction generally perpendicular to a plane in which a surface of the substrate is located. The aperture then is filled with conductive material, such as a metal, a metal alloy, doped polysilicon, or a conductive or conductor-filled resin, such as an epoxy, thereby forming a conductive via that provides a conductive path extending at least partially through the substrate.
In some cases, the aperture length is significantly greater than the diameter or lateral extent (if not circular) thereof. In such cases, the aperture is said to have a “high” aspect ratio. Filling an aperture having a relatively high aspect ratio with conductive material generally is more difficult than filling an aperture having a relatively low aspect ratio.
Where the substrate itself is electrically conductive or semiconductive, the walls of the substrate within the aperture must be electrically insulated prior to filling the aperture with conductive material to avoid short-circuiting. For example, a precursor aperture may be formed using, for example, a laser beam, an electron beam, an ion beam, or a very small drill bit or router bit. The precursor aperture is larger in diameter than the conductive via to be formed in the precursor aperture. After forming the precursor aperture, a thin layer of insulating material is formed or disposed on the surface of the substrate within the precursor aperture. In some instances, the precursor aperture then may be substantially filled with an electrically insulating polymer material, after which a subsequent, smaller aperture may be formed through the polymer material. In either case, the resulting, insulated aperture then may be filled with conductive material, as previously discussed.
To fill an aperture extending through a substrate with conductive solder material, the surfaces of the substrate adjacent and within the aperture generally are lined with a flux material, which may include an organic acid that prevents the formation of oxides and promotes wetting of the surfaces of the substrate by the solder material. Organic components of the flux material may volatilize upon heating of the substrate to generate gases, which may create voids within the conductive solder material or cause other defects in the substrate through which the conductive via extends.